The present invention relates to a catalyst and its use in a catalytic cracking process. More particularly, this invention relates to a zeolite containing cracking catalyst in combination with an alkaline earth metal phosphate and its use in a catalytic cracking zone.
In commercial catalytic cracking processes, a trend in recent years has been to use heavier feed stocks which contain more Conradson carbon precursors and metal contaminants such as nickel, vanadium, and iron. As a result, more coke is made in the catalytic cracking zone, leading to higher regenerator temperatures and requiring in some cases the use of catalyst coolers. In addition to excessive coke making, other factors such as metal contaminants lead to deactivation and destruction of the active zeolite component. Product selectivity also suffers, leading to more gas production, especially hydrogen, and lower gasoline yield.
To overcome these adverse effects, at least partially, alumina is frequently added to the catalyst mixture in the cracking zone to assist in bottoms conversion, and increase the catalyst material's resistance to nitrogen and metals. However, coke making is increased. It has previously been found that monoammonium phosphate (MAP) treatment of the alumina is effective in lowering coke while still maintaining the desirably resistance to the aforementioned contaminants. While the MAP treatment is a desirable improvement in cracking heavy feed, some disadvantages accrue since MAP is soluble and only a portion is deposited on the catalyst requiring the manufacturer to employ additional steps to recover the remaining phosphate from the effluent plant streams. Also, the MAP treatment has been found to form a catalyst which is less resistant to attrition.
There is therefore a need for a catalyst for the cracking of petroleum based feedstocks which makes less coke, has increased metals resistance, exhibits lower attrition, and is easier to manufacture.
Hydrocarbon cracking catalysts comprising a zeolite and discrete particles of alumina dispersed in an inorganic oxide matrix are known. See, for example, U.S. Pat. No. 4,283,309 and U.S. Pat. No. 4,259,212. Although the added alumina particles, which in themselves prior to being composited with the other components have relatively little cracking activity, the catalysts comprising the added alumina particles have increased activity, increased vanadium resistance, and increased bottoms conversion. However, such catalysts exhibit undesired coke production.
U.S. Pat. Nos. 4,584,091 and 4,657,152 to Pine disclose that by treating the alumina particles with certain phosphorus compounds prior to compositing the alumina particles with the other catalysts or catalysts precursor components, the catalyst comprising the phosphorus-treated alumina particles has increased selectivity for naphtha components and produces less coke and gas.
U.S. Pat. No. 4,454,241 discloses a phosphorus containing zeolitic catalyst made from a clay starting material. The catalyst is obtained by contacting a partially cation exchanged calcined zeolite-containing catalyst with a dihydrogen phosphate anion, e.g. ammonium hydrogen phosphate or dihydrogen phosphite anion.
U.S. Pat. No. 3,507,778 discloses a zeolite in combination with a phosphate promoted silica-magnesia catalyst for cracking petroleum fractions. Example 4 discloses an ammonium phosphate.
U.S. Pat. No. 4,228,036 discloses a cracking catalyst comprising an alumina-aluminum phosphate silica matrix composited with a zeolite.
U.S. Pat. No. 4,179,358 discloses a cracking catalyst comprising zeolite dispersed in a magnesia-alumina-aluminum phosphate matrix.
U.S. Pat. No. 4,430,199 discloses passivation of contaminant metals on cracking catalysts by phosphorous addition. The phosphorus compound may be ammonium hydrogen phosphate. The phosphorus compound may be impregnated on an inert carrier such as calcined metakaolin clay that can be blended with the catalyst or added to the catalyst. In particular, reference is made to column 3, line 17 to 20 and column 10, lines 20 to 25.